The description of a new skull from the Dmanisi site (Republic of Georgia) concludes that the site hosted not one but two Homo species, one living around 1.8 million years ago and another several hundred thousand years later.
The Pleistocene site has yielded an impressive hominin fossil assemblage, opening fresh perspectives for understanding the nature of the first Eurasian human settlers, and providing important data for reassessing of the origin and evolution of the genus.
Based on one of the lower jaws, which is considerably larger than the others recovered from the site and fits with the newly described skull, the study describes remarkable shape differences that do not depend on body size or sex, stating that the large fossil exhibits a mosaic of primitive and derived features absent from the smaller specimens, signaling the presence of separate species. The small jaws come from a population that was closely related to early African Homo populations, the scientists conclude, suggesting the larger jaw belonged to Homo georgicus, a poorly understood species.
Excavation director David Lordkipanidze of the Georgian National Museum in Tbilisi disagrees, believing that shape similarities among skulls that fit the lower jaws indicate that only one Homo species occupied the site. Geologic studies show that the Dmanisi fossils are no younger than 1.76 million years old, he adds, whereas the new study's revision of the complex stratigraphy suggests the accumulation could cover an undetermined period of time.
Most researchers acknowledge the high degree of size and shape differences at Dmanisi, although their interpretations differ.
According to Lordkipanidze and his team, the large variability exhibited by the Dmanisi hominins would lessen the differences used to identify species such as Homo habilis, Homo rudolfensis, Homo ergaster or Homo erectus. All of these would thus belong to the same species, representing regional variants of a single lineage that would have inhabited the Eurasian and African continents during a considerable large period.
However, if they belong to the same lineage, Dmanisi hominins would exhibit a sexual size difference greater than that observed in modern humans and chimpanzees.
The new study's authors expect that future discoveries at Dmanisi and revisions of the fossil record will shed light on the interpretation of these hominins, saying the evidence available at present suggests the first dispersion out of Africa was probably more complex than previously supposed, that different ecological niches may have been present in the area where the fossils were found, and that the possibility of there having been two species should be further explored.

Edited from Plos One (20 February 2014), ScienceNews (27 February 2014)